1. Field of the Invention
The present invention relates to a method and an apparatus for superfinishing convex or concave jacket faces of rotationally symmetrical workpieces, in particular of roller bearing rollers. The workpiece is received and rotated between two rotatable rollers, at least one of which is driven, while a honing stone is lowered perpendicularly to the longitudinal axis of the workpiece against the jacket face and thereby executes an oscillation parallel to the longitudinal axis of the workpiece.
2. Prior Art
In this known method, the workpiece 3 is received, as shown in FIG. 1, between two rollers 1 and 2, which are rotating in the same rotational direction, and are coupled with them in the rotational direction as a result of friction. Because of a slightly oblique positioning of the rollers 1, 2 relative to one another, which is highly exaggerated in FIG. 1, a forward movement is simultaneously effected in the direction of the longitudinal axis of the workpiece. The workpiece 3 thereby travels through the apparatus underneath the honing stone 4, the honing stone 4 is lowered against the workpiece 3 in the direction indicated by the arrow shown above the honing stone holder 5 and pressed against the workpiece. As the workpiece 3 rotates, the honing stone 4, driven by a mechanism which is not shown, executes an oscillation having a frequency of 1500 Hz, for example, in the direction of the longitudinal axis of the workpiece 3. This machining operation on the jacket face of the workpiece, which is also known as superhoning, exterior fine honing or superfinishing, produces an extraordinarily smooth and dense surface; at the same time, errors in concentricity of the workpiece are compensated for.
A workpiece shape such as that shown in FIG. 2 is frequently desirable. This shape is found in roller bearing rollers the jacket faces of which are cylindrical in the middle, in the zone marked Z, and then rounded (that is, convex) in the adjoining zones B, where the radius of curvature decreases in length from a value of R.sub.max adjoining zone Z to a value of R.sub.min at the end. With such shapes it is possible to avoid peaks in mechanical strain which would otherwise occur at the ends and which cause great and irregular strains and wear in roller bearing operation, especially if the roller bearing rollers are installed in even a slightly crossed position. The ideal shape of the workpiece 3 of FIG. 2 has a continuous transition between zone Z and zone B; that is, the cylindrical jacket line in zone Z is a tangent to the adjacent rounded or convexly curved zone B. Furthermore, the radius of curvature of the rounded or convex part B should become steadily smaller (that is, the curvature should be increased) toward the end of the roller bearing rollers, until at the end of the roller bearing roller the curvature is at its greatest. The shape of the rounded part or convex zone B is accordingly parabolic.
Thus far it has not been possible to meet these demands with a method or an apparatus according to FIG. 1. It was only possible to fabricate roller bearing rollers such as that shown in principle in FIG. 3, that is, having a single radius of curvature--rather than a radius of curvature which decreases steadily toward the outer end--and with a discontinuous or abruptly angled transition between the cylindrical zone Z and the rounded zone B. This was the result of the only manner in which it was possible to fabricate these kind of jacket faces in roller bearing rollers. That is, with an apparatus such as that shown in FIG. 1, the rollers 1 and 2 are not ground precisely cylindrically, but rather such that the outer contour is given practically a "hill", that is, an elevation, such as that shown in FIG. 4 and marked 2'. Since, as has already been noted, an advancing movement is imparted to the roller bearing roller in the direction of its longitudinal axis as a result of the oblique positioning of the rollers 1, 2 relative to one another, the roller travels over this elevated contour 2' as well, and the result, in sequence, are the various positions 3-1, 3-3, producing a circularly rounded shape at the ends as a result of the machining performed with the honing stone 4. However, this operation produces only a single, definite radius of curvature and a discontinuous transition from Z to B.
A further disadvantage of the known method (FIGS. 1, 3, 4) is that only radii of curvature up to a certain limit can be attained. If smaller radii of curvature are desired, then the elevated contour 2' would have to be so pronounced that there would no longer be any force component in the advancement direction. Thus the continuous-travel method described above fails completely.
A further and particularly grave disadvantage of the known method is that it is extraordinarily difficult to fabricate rollers having contours such as are shown in FIG. 4. First they must be calculated in an extremely complicated manner, and then they must be ground in an operation of great difficulty.